Ink jet printing processes are mainly of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing systems, a continuous ink stream is emitted under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through an electric static field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets are collected in the collecting vessel.
In the non-continuous process, or the so-called “drop-on-demand” systems, a droplet is generated and expelled from the nozzle in accordance with digital data signals only if a specific location on the recording sheet has to be printed.
The printing speed of modern ink jet printers is always increasing for economical reasons. Recording sheets suitable for these printers therefore need to absorb the inks very quickly. Especially suitable are recording sheets containing nanocrystalline, nanoporous inorganic oxides, preferably aluminum oxides or aluminum oxide/hydroxides.
Such recording sheets available today do not meet all of the required demands. In particular, the light stability and the storage stability of images printed on these recording sheets have to be improved. These images are not particularly stable when they are in contact with ambient air, which normally contains sulfur dioxide and, especially in summer, photochemically generated impurities such as ozone or nitrogen oxides. The images are strongly altered or even destroyed in a short time when they are in contact with ambient air. These phenomena are described for example in “Ozone Problem with Epson Photo Paper”, Hardcopy Supplies Journal 6 (7), 35-36 (2000).
In patent application EP 0,373,573 derivatives of polyhydroxybenzenes are proposed as stabilizers for recording sheets for ink jet printing.
Patent application EP 0,534,634 describes the deposition of salt solutions containing at least one bivalent metal cation onto recording sheets for ink jet printing in order to improve the water fastness of the printed images. The copper salts CuCl2, CuBr2, Cu(NO3)2, Cu(ClO3)2 and Cu(C2H3O2)2 are mentioned explicitly.
Patent application JP 1-301,359 describes the addition of organic sulfonates or organic sulfates in combination with copper or nickel salts of monocarboxylic acids to recording sheets for ink jet printing in order to improve the light stability of the printed images. The copper salts copper formiate and copper acetate are mentioned explicitly.
In patent application GB 2,088,777 derivatives of phenols and bisphenols are proposed in order to improve the stability of recording sheets containing nanoporous inorganic oxides or oxide/hydroxides.
In patent application EP 0,685,345 the addition of dithiocarbamates, thiocyanates, thiurams or sterically hindered amines to recording sheets containing nanoporous inorganic oxides or oxide/hydroxides is proposed in order to improve their stability.
Patent application WO 00/37,574 describes the addition of bivalent salts of carboxylic acids with at least 4 carbon atoms of copper, nickel, cobalt or manganese to inks as well as to recording sheets for ink jet printing in order to improve the light stability of the printed images. It is mentioned that the bivalent copper salts may be replaced by monovalent copper salts.
Patent application EP 1,197,345 describes the addition of unsubstituted or substituted 1,3-cyclohexanedione to recording sheets for ink jet printing containing nanoporous inorganic oxides or oxide/hydroxides in order to increase the stability of printed images when these are in contact with contaminated ambient air.
Patent application EP 1,231,071 proposes the addition of the salts copper(I) chloride, copper(I) bromide or copper(I) sulfite monohydrate of monovalent copper to recording sheets for ink jet printing containing nanoporous inorganic oxides or oxide/hydroxides in order to increase the stability of printed images when these are in contact with contaminated ambient air.
Patent application EP 1,262,329 discloses that poly(aluminum hydroxychloride) may be added to recording sheets for ink jet printing containing aluminum oxide/hydroxide, in particular pseudo-boehmite. Aluminum oxide/hydroxide, as disclosed in patent applications JP 02-276,670, JP 03-067,684, JP 03-251,488, (ICH-302 CIP) JP 04-067,986, JP 04-263,983 and JP 05-016,517 may be used as nanoporous inorganic compound in the ink-receiving layer.
These patent applications disclose the preparation of aluminum oxide/hydroxide from aluminum alkoxides in the presence of acids. In particular, patent application JP 05-016,517 discloses the preparation of aluminum oxide/hydroxide from aluminum isopropoxide in the presence of acetic acid.
All these proposed additives however do not sufficiently increase the stability of printed images on recording sheets for ink jet printing containing nanocrystalline, nanoporous inorganic oxides or oxide/hydroxides when these are in contact with contaminated ambient air. In particular, all reducing additives are quickly oxidized by oxygen or the impurities contained in the ambient air and therefore rapidly loose their stabilizing behavior. Some of the proposed additives may also be transformed into colored compounds when they are in contact with ambient air, leading to an unwanted degradation of the brightness of the recording sheets or of the images printed thereon.
The colloidal, nanoporous aluminum oxide/hydroxide that is very often used in recording sheets is normally prepared in the Yoldas sol-gel process by the hydrolysis of aluminum alkoxides, as described in the book by C. F. Brinker and G. W Scherer, “Sol-Gel Science”, Academic Press, 1990, ISBN 0-12-134970-5, pages 59-78. The addition of an aqueous acid, for example hydrochloric acid, nitric acid, acetic acid or lactic acid, is always a step during the preparation process, either before, during or after hydrolysis of the aluminum alkoxides. Otherwise no clear sol is obtained.
This process is described by B. E. Yoldas in “A Transparent Porous Alumina Sol”, American Ceramic Society Bulletin 54, 286-288 (1975), B. E. Yoldas in “Alumina Sol Preparation from Alkoxides”, American Ceramic Society Bulletin 54, 289-290 (1975) and B. E. Yoldas in “Alumina gels that form porous transparent Al2O3”, Journal of Materials Science 10, 1856-1860 (1975). After hydrolysis, the solvents of the reaction mixture are evaporated in order to isolate the nanocrystalline, nanoporous aluminum oxide/hydroxide as a solid.
The preparation of colloidal boehmite rods is described by M. P. B van Bruggen in “Liquid Crystal Formation and Diffusion in Dispersions of Colloidal Rods”, 1998, ISBN 90-393-1987-1, page 59. Here, hydrochloric acid is used in the hydrolysis mixture of aluminum-iso-propoxide and aluminum-sec-butoxide.
Patent DE 3,823,895, on the other hand, describes a process for the preparation of nanocrystalline, nanoporous aluminum oxide/hydroxide, wherein the whole preparation process takes place in the absence of acids, before, during or after hydrolysis of the aluminum alkoxides.
Preferably the nanocrystalline, nanoporous aluminum oxide/hydroxide contains one or more elements of the rare earth metal series of the periodic system of the elements as described in patent application EP 0,875,394.
The treatment of colloidal aluminum oxide or aluminum oxide/hydroxide prepared in the presence of acids with aluminum chlorohydrate is described by M. P. B van Bruggen in “Liquid Crystal Formation and Diffusion in Dispersions of Colloidal Rods”, 1998, ISBN 90-393-1987-1, pages 58-79. The aluminum chlorohydrate is added as a solid to the dispersion of aluminum oxide or aluminum oxide/hydroxide.
There is therefore still a need to improve, in addition to the ink absorption capacity, the image quality, the water fastness, the light stability, the storage stability of printed recording sheets containing nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide when they are in contact with ambient air containing impurities such as ozone, nitrogen oxides or sulfur dioxide and, in particular bronzing. “Bronzing” is the formation of dye deposits with a metallic shiny gloss on the surface of a printed recording sheet. A printed recording sheet with bronzing shows different colors when viewed at different viewing angles. Dyes from aqueous inks do not penetrate into the interior of the recording sheet, but are deposited on the surface, deteriorating image quality and smudge behavior. Bronzing is in most cases more pronounced in printed recording sheets that have been exposed to light than in freshly printed recording sheets.